On the '"Tauchschiff Projekt 50", 1918' (http://dreadnoughtproject.org/plans/SM_Projekt_50_1918/) web page I noticed the emphasis on the main deck guns being 150 mm. Something looked wrong so I decided to check. Using the scale along the keel the length of the these guns are around 11 m which for a 15 cm gun is a calibre of around 73! During WW2 I gather there was a German tank gun that had a calibre of 70 but for WW1 naval guns the usual limit appears to be 45 - which is 6.71 m long (https://en.wikipedia.org/wiki/15_cm_SK_L/45).

I found a drawing of a 15cm L/45 naval gun (http://www.navweaps.com/Weapons/WNGER_59-45_skc16.jpg) and calculated the outside muzzle diameter of 24 cm - while the Projekt 50 deck gun is around 37 cm.

When I looked at the "List of NAVAL guns by country" (https://en.wikipedia.org/wiki/List_of_naval_guns_by_country) web page I came across a perfect fit - a 28cm SK L/40 beast (https://en.wikipedia.org/wiki/28_cm_SK_L/40_gun). It was 11.2m long (does that ring a bell?) and used on two classes of pre-dreadnought battleships. The clincher was that the Battle of Jutland in 1916 showed that these ships were obsolete so their guns were to be removed and adapted for land service.

As far as the Imperial German Navy was concerned what better home for them could there be then usefully reused - mounted on a monstrous steam powered "Tauchschiff"?

Therefore in conclusion it appears that the main deck guns were not 150 mm but 280 mm, which would have come as a nasty surprise. Finally consider that the standard maximum gun for even WW2 subs was 150 mm - only exceeded by the French 'Surcouf' with two 203 mm or the British 'M2' mighty 305 mm!

I daresay one problem with it is weight. The weight of the WWI German 15cm L/45 C/16 gun which armed "large World War I destroyers...and U-cruisers", which I think would be the gun which this monster would armed with is 3,990kg

The weight of a German 28cm SK L/40 C/01 gun, which armed German Braunschweig and Deutschland class battleships was 45,300kg. Four of these guns would be 181,200kg on the deck of this submarine. I am not sure that this submarine, the monster that is, could carry this much weight on its deck and remain stable and upright.

FWIW, there was a later German 28cm gun, the 28cm L/45 C/07 gun which armed the Nassau class battleships and the battlecruiser Von Der Tann, which weighed 39,800kg and some of these were available for use on land.

I have done what I should have done in the first place, checked the German sources (albeit in English)

THE U-BOAT, by Eberhard Rossler (1981 English edition)

As to 'Project 50' (or K44),

"The torpedo armament corresponded to that of Project 46A. The guns consisted of 15cm quick-firing guns, two forward and one abaft the conning tower, and two 8.8cm AA guns. On 24 November 1917, the Weapons Department ('W') demanded four 15cm quick-firing guns, which the UI rejected as impossible because of weight problems, and suggested instead four guns in twin mountings. 'W' refused to consider twin mountings for U-boats, and insisted on four QF guns - if need be, at the expense of the AA guns. After further work on the project, it was decided that the firm of Erhardt should provide the new, hydraulically-operated 15cm L/45 U-boat gun in a special submarine mounting. The question as to whether three or four 15cm guns would be used was not resolved."

GERMAN WARSHIPS 1815-1945 VOLUME TWO U-BOATS AND MINE WARFARE VESSELS, by Erich Groner, revised and expanded by Dieter Jung and Martin Maas (1991 English edition)

Entry for "U1 Armoured U-cruiser design" UD1

"official Design Project 50, 1917"

"Armament. Probably six 50cm torpedo tubes, four at the bow below the CWL and two at the stern below the CWL - number of torpedoes unknown. Three or four Utof 15cm/45 guns - number of rounds unknown.

Interestingly, in view of the plan posted on the website, Groner says that "The original plans, by the engineer H Wolke, were destroyed in1946, and it is unlikely that any other official documents still exist."

Apparently Groner was not correct or the plan was located after his death.

So, I am afraid the theory set forth by the original poster was pure speculation and not correct.

It is not surprising that Groner did not see the plan I have, as it was taken by the Soviets at the end of WW2. All my plans came to me through Russian hands, likely taken from Soviet archives near St. Petersburg while the repository was being prepared for repatriation to Germany in the 1990s.

I am not sure whether this is the only known one depicting this project (or, this incarnation of it), but it may be.

Mine also shows swivelling twin torpedo tubes able to fire to either 45 degrees red or green, not mentioned in your cited accounts, and a pair of fixed aft tubes on the diagnoal quarters, giving this design 4 forward, two on swivel mount and 4 aft (two on quarter, two firing rearward).